Hierarchical Modeling I

Hierarchical Modeling I
Ed Angel Professor of Computer Science, Electrical and Computer Engineering, and Media Arts University of New Mexico

Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005
Objectives Examine the limitations of linear modeling Symbols and instances Introduce hierarchical models Articulated models Robots Introduce Tree and DAG models Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Objectives Build a tree-structured model of a humanoid figure Examine various traversal strategies Build a generalized tree-model structure that is independent of the particular model Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Objectives Introduce graphical objects Generalize the notion of objects to include lights, cameras, attributes Introduce scene graphs Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Instance Transformation
Start with a prototype object (a symbol) Each appearance of the object in the model is an instance Must scale, orient, position Defines instance transformation Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Symbol-Instance Table
Can store a model by assigning a number to each symbol and storing the parameters for the instance transformation Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Relationships in Car Model
Symbol-instance table does not show relationships between parts of model Consider model of car Chassis + 4 identical wheels Two symbols Rate of forward motion determined by rotational speed of wheels Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Structure Through Function Calls
car(speed) { chassis() wheel(right_front); wheel(left_front); wheel(right_rear); wheel(left_rear); } Fails to show relationships well Look at problem using a graph Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Graphs Set of nodes and edges (links) Edge connects a pair of nodes Directed or undirected Cycle: directed path that is a loop loop Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Tree Graph in which each node (except the root) has exactly one parent node May have multiple children Leaf or terminal node: no children root node leaf node Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Tree Model of Car Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

DAG Model If we use the fact that all the wheels are identical, we get a directed acyclic graph Not much different than dealing with a tree Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Modeling with Trees Must decide what information to place in nodes and what to put in edges Nodes What to draw Pointers to children Edges May have information on incremental changes to transformation matrices (can also store in nodes) Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Robot Arm parts in their own coodinate systems robot arm Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Articulated Models Robot arm is an example of an articulated model Parts connected at joints Can specify state of model by giving all joint angles Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Relationships in Robot Arm
Base rotates independently Single angle determines position Lower arm attached to base Its position depends on rotation of base Must also translate relative to base and rotate about connecting joint Upper arm attached to lower arm Its position depends on both base and lower arm Must translate relative to lower arm and rotate about joint connecting to lower arm Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Required Matrices Rotation of base: Rb Apply M = Rb to base Translate lower arm relative to base: Tlu Rotate lower arm around joint: Rlu Apply M = Rb Tlu Rlu to lower arm Translate upper arm relative to upper arm: Tuu Rotate upper arm around joint: Ruu Apply M = Rb Tlu Rlu Tuu Ruu to upper arm Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

OpenGL Code for Robot robot_arm() { glRotate(theta, 0.0, 1.0, 0.0); base(); glTranslate(0.0, h1, 0.0); glRotate(phi, 0.0, 1.0, 0.0); lower_arm(); glTranslate(0.0, h2, 0.0); glRotate(psi, 0.0, 1.0, 0.0); upper_arm(); } Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Tree Model of Robot Note code shows relationships between parts of model Can change “look” of parts easily without altering relationships Simple example of tree model Want a general node structure for nodes Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Possible Node Structure
Code for drawing part or pointer to drawing function linked list of pointers to children matrix relating node to parent Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Generalizations Need to deal with multiple children How do we represent a more general tree? How do we traverse such a data structure? Animation How to use dynamically? Can we create and delete nodes during execution? Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Humanoid Figure Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Building the Model Can build a simple implementation using quadrics: ellipsoids and cylinders Access parts through functions torso() left_upper_arm() Matrices describe position of node with respect to its parent Mlla positions left lower leg with respect to left upper arm Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Tree with Matrices Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Display and Traversal The position of the figure is determined by 11 joint angles (two for the head and one for each other part) Display of the tree requires a graph traversal Visit each node once Display function at each node that describes the part associated with the node, applying the correct transformation matrix for position and orientation Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Transformation Matrices
There are 10 relevant matrices M positions and orients entire figure through the torso which is the root node Mh positions head with respect to torso Mlua, Mrua, Mlul, Mrul position arms and legs with respect to torso Mlla, Mrla, Mlll, Mrll position lower parts of limbs with respect to corresponding upper limbs Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Stack-based Traversal
Set model-view matrix to M and draw torso Set model-view matrix to MMh and draw head For left-upper arm need MMlua and so on Rather than recomputing MMlua from scratch or using an inverse matrix, we can use the matrix stack to store M and other matrices as we traverse the tree Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Traversal Code figure() { glPushMatrix() torso(); glRotate3f(…); head(); glPopMatrix(); glPushMatrix(); glTranslate3f(…); left_upper_arm(); save present model-view matrix update model-view matrix for head recover original model-view matrix save it again update model-view matrix for left upper arm recover and save original model-view matrix again rest of code Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Analysis The code describes a particular tree and a particular traversal strategy Can we develop a more general approach? Note that the sample code does not include state changes, such as changes to colors May also want to use glPushAttrib and glPopAttrib to protect against unexpected state changes affecting later parts of the code Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

General Tree Data Structure
Need a data structure to represent tree and an algorithm to traverse the tree We will use a left-child right sibling structure Uses linked lists Each node in data structure is two pointers Left: next node Right: linked list of children Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Left-Child Right-Sibling Tree
Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Tree node Structure At each node we need to store Pointer to sibling Pointer to child Pointer to a function that draws the object represented by the node Homogeneous coordinate matrix to multiply on the right of the current model-view matrix Represents changes going from parent to node In OpenGL this matrix is a 1D array storing matrix by columns Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

C Definition of treenode
typedef struct treenode { GLfloat m[16]; void (*f)(); struct treenode *sibling; struct treenode *child; } treenode; Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Defining the torso node
treenode torso_node, head_node, lua_node, … ; /* use OpenGL functions to form matrix */ glLoadIdentity(); glRotatef(theta[0], 0.0, 1.0, 0.0); /* move model-view matrix to m */ glGetFloatv(GL_MODELVIEW_MATRIX, torso_node.m) torso_node.f = torso; /* torso() draws torso */ Torso_node.sibling = NULL; Torso_node.child = &head_node; Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Notes The position of figure is determined by 11 joint angles stored in theta[11] Animate by changing the angles and redisplaying We form the required matrices using glRotate and glTranslate More efficient than software Because the matrix is formed in model-view matrix, we may want to first push original model-view matrix on matrix stack Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Preorder Traversal void traverse(treenode *root) { if(root == NULL) return; glPushMatrix(); glMultMatrix(root->m); root->f(); if(root->child != NULL) traverse(root->child); glPopMatrix(); if(root->sibling != NULL) traverse(root->sibling); } Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Notes We must save model-view matrix before multiplying it by node matrix Updated matrix applies to children of node but not to siblings which contain their own matrices The traversal program applies to any left-child right-sibling tree The particular tree is encoded in the definition of the individual nodes The order of traversal matters because of possible state changes in the functions Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Dynamic Trees If we use pointers, the structure can be dynamic typedef treenode *tree_ptr; tree_ptr torso_ptr; torso_ptr = malloc(sizeof(treenode)); Definition of nodes and traversal are essentially the same as before but we can add and delete nodes during execution Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Limitations of Immediate Mode Graphics
When we define a geometric object in an application, upon execution of the code the object is passed through the pipeline It then disappears from the graphical system To redraw the object, either changed or the same, we must reexecute the code Display lists provide only a partial solution to this problem Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

OpenGL and Objects OpenGL lacks an object orientation Consider, for example, a green sphere We can model the sphere with polygons or use OpenGL quadrics Its color is determined by the OpenGL state and is not a property of the object Defies our notion of a physical object We can try to build better objects in code using object-oriented languages/techniques Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Imperative Programming Model
Example: rotate a cube The rotation function must know how the cube is represented cube data Application glRotate results Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Object-Oriented Programming Model
In this model, the representation is stored with the object The application sends a message to the object The object contains functions (methods) which allow it to transform itself Application Cube Object message Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

C/C++ Can try to use C structs to build objects C++ provides better support Use class construct Can hide implementation using public, private, and protected members in a class Can also use friend designation to allow classes to access each other Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Cube Object Suppose that we want to create a simple cube object that we can scale, orient, position and set its color directly through code such as cube mycube; mycube.color[0]=1.0; mycube.color[1]= mycube.color[2]=0.0; mycube.matrix[0][0]=……… Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Cube Object Functions We would also like to have functions that act on the cube such as mycube.translate(1.0, 0.0,0.0); mycube.rotate(theta, 1.0, 0.0, 0.0); setcolor(mycube, 1.0, 0.0, 0.0); We also need a way of displaying the cube mycube.render(); Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Building the Cube Object
class cube { public: float color[3]; float matrix[4][4]; // public methods private: // implementation } Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

The Implementation Can use any implementation in the private part such as a vertex list The private part has access to public members and the implementation of class methods can use any implementation without making it visible Render method is tricky but it will invoke the standard OpenGL drawing functions such as glVertex Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Other Objects Other objects have geometric aspects Cameras Light sources But we should be able to have nongeometric objects too Materials Colors Transformations (matrices) Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Application Code cube mycube; material plastic; mycube.setMaterial(plastic); camera frontView; frontView.position(x ,y, z); Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Light Object class light { // match Phong model public: boolean type; //ortho or perspective boolean near; float position[3]; float orientation[3]; float specular[3]; float diffuse[3]; float ambient[3]; } Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Scene Descriptions If we recall figure model, we saw that We could describe model either by tree or by equivalent code We could write a generic traversal to display If we can represent all the elements of a scene (cameras, lights,materials, geometry) as C++ objects, we should be able to show them in a tree Render scene by traversing this tree Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Preorder Traversal glPushAttrib glPushMatrix glColor glTranslate glRotate Object1 Object2 glPopMatrix glPopAttrib … Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Separator Nodes Necessary to isolate state chages Equivalent to OpenGL Push/Pop Note that as with the figure model We can write a universal traversal algorithm The order of traversal can matter If we do not use the separator node, state changes can propagate Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

Inventor and Java3D Inventor and Java3D provide a scene graph API Scene graphs can also be described by a file (text or binary) Implementation independent way of transporting scenes Supported by scene graph APIs However, primitives supported should match capabilities of graphics systems Hence most scene graph APIs are built on top of OpenGL or DirectX (for PCs) Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

VRML Want to have a scene graph that can be used over the World Wide Web Need links to other sites to support distributed data bases Virtual Reality Markup Language Based on Inventor data base Implemented with OpenGL Angel: Interactive Computer Graphics 4E © Addison-Wesley 2005

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